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ORNITOLOGIA NEOTROPICAL 12: 271–275, 2001
© The Neotropical Ornithological Society
FORAGING AT ARMY ANT SWARMS BY FIFTY BIRD SPECIES IN
THE HIGHLANDS OF COSTA RICA
Andrew C. Vallely
99 Clove Avenue, Haverstraw, New York 10927, USA. E-mail: [email protected]
Forrajeo en ejambres de hormigas soldado por cincuenta especies de aves en las tierras
altas de Costa Rica.
Key word: Ant-following birds, Costa Rica.
Much has been written concerning the use of
raiding swarms of army ants by Neotropical
birds in lowland forests. Bird-ant foraging
associations are much less common at higher
(subtropical) elevations, and no highland species have been identified as professional or
regular ant-followers. All known professional
army ant followers in the Neotropics occur
below 1000 meters elevation (Ridgely &
Tudor 1994). Even attendance at army ant
swarms by montane birds has seldom been
reported, and previous authors have
described observations made at a single, or at
most six, highland army ant swarms (Hilty
1974, Gochfeld & Tudor 1978 and Dobbs &
Martin 1998). In this note, I report observations of 50 species of birds attending 17 separate raiding army ant swarms in subtropical
forests of the Cordillera de Tilaran at Monteverde, Costa Rica.
During January, November and December 1996, and from January through May
1997, I made opportunistic observations of
birds at army ant swarms in the vicinity of
Monteverde. Elevations were estimated using
topographical maps. Army ant swarms were
encountered at sites ranging in elevation from
1100 to 1620 meters, on both the east and
west slopes of the Cordillera de Tilaran. Thirteen swarms were found inside forest and
four were found in edge (disturbed) habitats.
When birds were located feeding at an
army ant swarm, I recorded the species and
number of individuals present. All swarms
were watched for at least one hour to ensure
that all species and individuals using the
swarm were detected. Species were considered to be “ant following” if they remained
with the swarm for the total period of observation and were observed capturing, or
attempting to capture, prey flushed by the
advancing ants.
I recorded a total of 244 individuals at 17
separate army ant swarms. The total number
of individuals detected at any one swarm
ranged from 5 to 24 (average = 15.59, SD =
4.887). The number of species attending a
single swarm ranged from 3 to14 (average =
8.76, SD = 3.134). Of the 50 species at these
swarms, 20 have apparently not previously
been reported following ants (Table 1). Only
one species, Myrmeciza immaculata (present at
271
VALLELY
TABLE 1. Species recorded at highland army ant swarms.
Species
Geotrygon chiriquensis*
Eupherusa eximia*
Momotus momota
Premnoplex brunnescens
Margarornis rubiginosus*
Syndactyla subalaris
Thripadectes rufobrunneus*
Dendrocincla homochroa
Sittasomus grisecapillus
Xiphorhynchus erythropygius
Dysthamnus mentalis
Myrmotherula shisticolor
Myrmeciza immaculata
Formicarius rufipectus*
Mionectes olivaceus*
Platyrinchus mystaceus
Empidonax flavescens*
Chiroxiphia linearis*
Cyanocorax morio
Cyanolyca cucullata*
Thryothorus modestus
T. rufalbus
Troglodytes aedon
T. ochraceous*
Henicorhina leucophrys
Myadestes melanops
Catharus aurantiirostris
C. fuscater
C. frantzii*
C. ustulatus
Hylocichla mustellina
Turdus assimilis
T. obsoletus
T. grayi
Vermivora chrysoptera*
Mniotilta varia
Seirus aurocapillus
Oporornis formosus
Wilsonia pusilla
Myioborus miniatus*
M. torquatus*
Basileuterus culcivorus
B. tristriatus*
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Number of swarms Average number of
(n = 17) where
individuals
present
1
1
6
7
2
1
1
1
1
2
2
1
5
1
1
2
3
1
3
3
2
3
1
1
9
2
3
10
4
2
3
2
1
2
1
1
1
2
2
3
4
5
7
1
1
2.3
1.42
1.5
1
1
1
1
1
1.5
2
2
2
1
1
1
1
1.6
1.6
2
2
1
1
1.8
1
1.3
3.1
1
1
1
2
1
1
1
1
1
1
1
1.3
2.25
1.8
2.4
SD**
0
0
.7
.4
.5
0
0
0
0
0
.5
0
.6
0
0
0
0
0
.4
.9
0
0
0
0
.3
0
.4
1.3
0
0
0
0
1
0
0
0
0
0
0
.4
.4
.4
1.1
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TABLE 1. Continuation.
Species
Number of swarms Average number of
(n = 17) where
individuals
present
Chlorospingus opthalmicus*
Tangara dowii*
Lysurus crassirostris
Pselliophorus tibialis*
Atlapetes albinucha*
Buarremon brunneinuchus
Melozone leucotis*
4
1
1
2
3
5
3
3.5
1
1
2
2
1.2
2
SD**
.86
0
0
0
0
.4
0
*Species not previously reported attending ant-swarms.
**SD = Standard deviation.
five swarms), has been considered to be a
regular ant-follower (Willis 1985b).
Willis & Oniki (1978) pointed out that
“irregular” or “casual” ant followers, dominate species lists in peripheral forests. My
observations support this conclusion in that
39 of the 50 species I recorded were present
at three or fewer of the 17 swarms surveyed,
including five species generally considered to
be frugivores; Geotrygon chiriquensis, Mionectes
olivaceous, Chiroxiphia linearis, Myadestes melanops
and Tangara dowii (Slud 1964). At one swarm I
observed a largely nectivorous species,
Eupherusa eximia, making short sallies, presumably for small insects. To the best of my
knowledge, this is the first observation of a
hummingbird foraging in association with an
army ant-swarm.
The ovenbirds Margarornis rubiginosus and
Thripadectes rufobrunneus were present at two
and one army ant swarms respectively. To the
best of my knowledge, this is the first report
of this behavior in either genus. Margarornis
rubiginosus is a frequent member of mixed
flocks (Slud 1964). Although I was able to
confirm that the individuals I watched were
feeding in direct association with army ant
swarms, it is possible that these birds were
initially attracted by the presence of other
mixed-flocking species, a behavior that Willis
(1983c) has termed “indirect” ant following.
I recorded two individuals of Formicarius
rufipectus at one swarm. This appears to be the
first record of this species foraging in association with army ants. Willis (1985a) noted that
three of the five other Formicarius species are
sometimes seen feeding at army ant swarms,
and commented that “antthrushes could easily follow ants regularly… it is not clear why
anttrushes are not good followers”.
Willis (1983a) considered ant following in
the Tyrannidae insignificant, but noted that
flycatchers sometimes follow ants well in the
highlands. Flycatchers (Tyrannidae) were not
well represented at the swarms I encountered. Empidonax flavescens was present at three
swarms. Willis (1983a) reported three other
Empidonax species attending army ant
swarms. Mionectes olivaceus was present at one
swarm. Willis noted occasional ant following
in Mionectes macconnelli (Willis (1983a).
A single male Chiroxiphia linearis was
present at one swarm. Willis (1984a) noted
two other Chiroxiphia species attending army
ant swarms, but remarked that for Pipridae in
general, leking activity, frugivory and other
factors must “…interfere with (ant) following”.
Hardy (1974) and Haemig (1989) have
both discussed the use of ant swarms by
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VALLELY
Neotropical jays. Willis (1983b) describes
observations of four species of Cyanocorax jays
attending army ant-swarms. Both Cyanocorax
morio and Cyanolyca cucullatta were present
(three observations each) at the swarms I
watched. This seems to be the first record of
ant-following for the genus Cyanolyca. As
Hardy remarked, jays are known for their
“omnivorous and enterprising behavior.”
Opportunistic use of ant swarms by jays is
not surprising, but it has rarely been noted.
In general, few New World wrens (Troglodytidae) are regularly recorded at army ant
swarms (Willis 1983d). A single individual of
Troglodytes ochraceous was present at one of the
swarms I watched. This seems to be the first
record of ant following in this mostly arboreal
(Slud 1964) species. The understory wren
Henicorhina leucophrys is known to follow army
ant swarms occaisionally (Gochfeld & Tudor
1978, Willis 1983d). Henichorhina leucophrys was
present at 9 (59%) of the swarms I watched,
including all swarms above 1400 meters elevation.
Willis (1966, 1978, 1984b) described the
Turdidae as “fairly regular” attendants at army
ant swarms, but noted that “New World
thrushes are mostly birds of montane or
semi-open habitats, in which ants are uncommon because of unfavorable climates” and
that “…several factors are unfavorable for
ground birds near ants”. My observations
suggest that at least some Catharus thushes
will follow army ants when they are present.
Catharus fuscater was the most commonly
encountered species at the swarms I
observed, being present at 10 (59%) of the
swarms. Among the species I observed, C. fuscater also had the largest number of individuals (2–10) at the swarms where I recorded it.
Taken together with C. frantzii and C. ustulatus
(generally replacing frantzii at lower elevations), resident Catharus thrushes were
present at 14 (82%) of the seventeen swarms
I encountered, including all swarms occurring
274
above 1400 meters. Catharus fuscater was generally the most active and aggressive species
at the swarms where it occurred. Groups of
two to 10 C. fuscater often occupied the area
around the central leading edge of an advancing swarm, presumably the most productive
zone for fleeing invertebrate prey. Individuals
of this species alternated between pursuing
prey on the ground near the lead edge of the
swarm, and making rapid, low flights to displace conspecifics. Intraspecific interactions
in this species were generally accompanied by
bouts of calling and wing-shivering.
Willis (1986) remarked that Basileuterus
warblers should be able to follow ants and he
detailed observastions of six lowland Basileuterus species at army ant swarms. Basileuterus
tristriatus, a highland species not mentioned by
Willis, was present at seven of the swarms I
watched. This species is a common member
of mixed flocks and this may be another
example of “indirect following” (Willis
1983c). Similarly, Myioborus warblers are also
frequent members of mixed species flocks.
These warblers seem not to have been previously recorded following army ant swarms.
Myioborus miniatus and M. torquatus were
present at three and four swarms respectively.
Melozone leucotis, Pselliophorus tibialis, and
Lysurus crassirostris, were present at three, two,
and one swarm respectively. All three species
are generally insectivorous and characteristically feed low in dense montane undergrowth
(Slud1964), and thus seem fairly well suited to
exploit army ant swarms. My observations
represent the first report of which I am aware
of army ant following in these montane genera.
The migrant warbler Vermivora chrysoptera
is seasonally fairly common at Monteverde,
but was present at only one of the army ant
swarms I watched. This species has not previously been recorded following army ants.
Willis (1966) has discussed at length the
use of army ant swarms, particularly those of
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Labidus praedator, by Nearctic-Neotropical
migrants. Seven (14%) of the 50 species I
recorded were Nearctic-Neotropical migrants
including two thrushes (Turdinae), and five
warblers (Parulinae). These were represented
by a total of 12 individuals, accounting for
only 5% of all individuals recorded at all the
swarms I observed. Migrants were generally
present only at lower elevation swarms in
edge habitats. Migrants accounted for 26% of
individuals at those swarms were they
occurred. Migrants were present at all four
swarms in edge habitats, but at only at two of
the swarms found in forest interiors.
The 50 species which I recorded attending army ant swarms includes canopy species,
species with flock-adapted feeding behaviors, as well as species which are not principally insectivorous. The use of army ant
swarms by highland birds is thought to represent a facultative, opportunistic use of a
scarce resource (Dobbs & Martin 1998). The
absence of competition from “professional”
ant followers at army ant swarms in highland
habitats likely allows the opportunistic use of
army ant swarms by species which lack the
behavioral specializations of lowland ant followers.
ACKNOWLEDGMENTS
I am grateful to Nickolas V. L. Brokaw, Elizabeth P. Mallory, and Guy Tudor, who all
made helpful comments on this manuscript.
Robert C. Dobbs and an anonymous
reviewer offered many helpful criticisms.
REFERENCES
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Gochfeld, M., & G. Tudor. 1978. Ant-following
birds in South American subtropical forests.
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Haemig, P. D. 1989. Brown Jays as army ant followers. Condor 91: 1008–1009.
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Accepted 4 April 2001.
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